Moltres has always been the easiest for me to catch. Articuno is the one I always had the most trouble with. Don't think I have ever caught Articuno on the first round before I lost all Pokemon or it fainted.

In HeartGold, on my way up to fight Red, I accidentally stumbled upon Moltres, thinking it was the path to Red. I chucked about 50 Ultra/Dusk Balls and a few others, the ball barely budged on any of those tries. I realized it was worthless, beat Red, came back later, and caught Moltres in a Poke Ball.

Articuno was always the easiest for me. :3 I noticed on every Firered run the bird trio match their Pokeballs. For Articuno it's the great ball, blue and blue. For Zapdos it was the Ultra ball, black, yellow and black, yellow (oh gosh reminds me about an annoying song -.-) and then Moltres and the Pokeball. The reason I catch Moltres in the Pokeball was I always ran out of the others. D: <